This invention relates to a process of regenerating spent HF-HNO.sub.3 pickle acid containing (ZrF.sub.6).sup.-2. It is particularly useful for regenerating spent acids employed to pickle zirconium based articles.
Mixed acids such as HF-HNO.sub.3 solutions are widely employed to pickle articles made of zirconium, titanium, iron and their alloys. As the pickling solutions etch the surfaces of the articles, the concentration of the etched metal eventually increases to a level where the pickling rate becomes unacceptably low. The spent acid solutions must then be regenerated by precipitating the metals to lower their concentrations in the solutions and, if necessary, then recharging by adding fresh HF and/or HNO.sub.3 to restore their nominal concentrations before being recycled to the pickling lines. Presently known regeneration processes are disclosed by U.S. Pat. Nos. 3,048,503; 4,105,469; 4,144,092; 4,330,342; 4,526,650 and 4,738,747 and also U.S. application Ser. No. 07/555,333, filed 07/19/90, (W.E. 54,825) on behalf of the assignee of the present invention, which are hereby incorporated by this reference.
U.S. Pat. Nos. 4,105,469 and 4,330,342 disclose processes for adding NaF to spent HF-HNO.sub.3 pickle acids containing (ZrF.sub.6).sup.-2 to precipitate Na.sub.2 ZrF.sub.6. U.S. Pat. No. 4,105,469 discloses the addition of crystalline NaF to spent acids. However, NaF is a rather expensive precipitant and has a low solubility. Insufficient additions of NaF may lead to the formation of NaZrF.sub.5.H.sub.2 O, which is a difficult-to-filter, gel-like material. On the other hand, excessive additions of NaF may lead to premature precipitation of Na.sub.2 ZrF.sub.6 because of the excess Na.sup.+ and F.sup.- in solution, which may interfere with the pickling step. U.S. Pat. No. 4,330,342 discloses the addition of NaOH solutions to the spent acids to generate NaF in solution rather than adding NaF in crystalline form. Such additions of highly basic solutions into highly acidic solutions must be carefully performed to prevent local corrosion or other damage to the processing equipment and injury to the operating personnel.
U.S. Ser. No. 07/555,333, (W.E. 54,825) discloses a process for adding Na.sub.2 SO.sub.4 and CaF.sub.2 to spent HF-HNO.sub.3 pickle acids containing (ZrF.sub.6).sup.-2 to precipitate Na.sub.2 ZrF.sub.6. First, the Na.sub.2 SO.sub.4 is added to precipitate Na.sub.2 ZrF.sub.6 and then the CaF.sub.2 is added to precipitate CaSO.sub.4 and to increase the F.sup.- concentration of the regenerated acid. Thus, this process produces two precipitates for disposal and introduces two additional process variables, i.e., Ca.sup.+2 ions and SO.sub.4.sup.-2 ions in the pickle acid. Also, he presence of SO.sub.4.sup.-2 ions or other extraneous ions in the system may require special treatment whether the pickle acid and/or Na.sub.2 ZrF.sub.6 is treated as a waste product or a useful material.